AD7745/AD7746

Rev. 0| Page 11 of 28

OUTPUT NOISE AND RESOLUTION SPECIFICATIONS

The AD7745/AD7746 resolution is limited by noise. The noise

performance varies with the selected conversion time.

Table 5 shows typical noise performance and resolution for the

capacitive channel. These numbers were generated from 1000

data samples acquired in continuous conversion mode, at an

connected only to the evaluation board (no external capacitors.)

Table 6 and Table 7 show typical noise performance and

resolution for the voltage channel. These numbers were

generated from 1000 data samples acquired in continuous

conversion mode with VIN pins shorted to ground.

RMS noise represents the standard deviation and p-p noise

represents the difference between minimum and maximum

results in the data. Effective resolution is calculated from rms

noise, and p-p resolution is calculated from p-p noise.

Table 5. Typical Capacitive Input Noise and Resolution vs. Conversion Time

Conversion

Time (ms)

Output Data

Rate (Hz)

–3dB Frequency

(Hz)

RMS Noise

(aF/√Hz)

RMS

Noise (aF)

P-P

Noise (aF)

Effective Resolution

(Bits)

P-P Resolution

(Bits)

11.0

90.9

87.2

4.3

40.0

212.4

17.6

15.2

11.9

83.8

79.0

3.1

27.3

137.7

18.2

15.9

20.0

50.0

43.6

1.8

12.2

82.5

19.4

16.6

38.0

26.3

21.8

1.6

7.3

50.3

20.1

17.3

62.0

16.1

13.8

1.5

5.4

33.7

20.5

17.9

77.0

13.0

10.5

1.5

4.9

28.3

20.7

18.1

92.0

10.9

8.9

1.5

4.4

27.8

20.8

18.2

109.6

9.1

8.0

1.5

4.2

27.3

20.9

18.2

Table 6. Typical Voltage Input Noise and Resolution vs. Conversion Time, Internal Voltage Reference

Conversion

Time (ms)

Output Data

Rate (Hz)

–3dB Frequency

(Hz)

RMS Noise

(µV)

P-P Noise

(µV)

Effective Resolution

(Bits)

P-P Resolution

(Bits)

20.1

49.8

26.4

11.4

62

17.6

15.2

32.1

31.2

15.9

7.1

42

18.3

15.7

62.1

16.1

8.0

4.0

28

19.1

16.3

122.1

8.2

4.0

3.0

20

19.5

16.8

Table 7. Typical Voltage Input Noise and Resolution vs. Conversion Time, External 2.5 V Voltage Reference

Conversion

Time (ms)

Output Data

Rate (Hz)

–3dB Frequency

(Hz)

RMS Noise

(µV)

P-P Noise

(µV)

Effective Resolution

(Bits)

P-P Resolution

(Bits)

20.1

49.8

26.4

14.9

95

18.3

15.6

32.1

31.2

15.9

6.3

42

19.6

16.8

62.1

16.1

8.0

3.3

22

20.5

17.7

122.1

8.2

4.0

2.1

15

21.1

18.3